1. Field of the Invention
This invention relates to vacuum vapor deposition of coatings onto substrates. More particularly the invention relates to a vapor source in which material for the coating is placed on a rotating hearth and evaporated by an electron beam.
2. Description of the Prior Art
There are many well-known techniques for coating a substrate by vacuum vapor deposition. Vapor sources allowing evaporation of a variety of materials have been developed. Particularly difficult problems are encountered when it is sought to vaporize a material, either a metal or a non-metal, which has a melting point of several hundred degrees. The principal problem is finding a suitable container which will not disintegrate and will not interact with the evaporant material and thereby contaminate the coating deposited on the substrate.
Electron beams are often employed in evaporation apparatus because they can be directed to a confined impact area on the surface of the target material to be evaporated. The maximum heating and the desired evaporation occur on the exposed surface of the evaporant material and not any part of the crucible or the surface of the material which is in contact with the crucible. Thus, the crucible can be cooled to minimize any reaction involving it. Greatly increased crucible life and greatly reduced contamination of the deposited coating are thus possible. A further advantage of the use of electron beams is close control of the evaporation process. Because the electron-beam power and impact area can be rapidly varied and closely regulated, the evaporant material temperature and the evaporation rate can be maintained at any desired level.
An electron beam is usually obtained by use of an electron gun comprising a heated filament and an anode. The thermally emitted electrons are accelerated to high velocities by applying a large potential difference between the filament and the anode. In order to reduce arcing between the closely spaced gun components, it is desirable to separate and isolate the electron gun from the streaming vapor generated by the evaporation of the target material. Effective separation can be obtained by placing the electron gun off to one side and preferably below the material to be evaporated. Some means, such as a permanent magnet and two planar pole pieces, are used to deflect the beam through a large angle, such as 180.degree. or 270.degree., before the beam is incident on the impact area. When 270.degree. deflection is used, a cover can be placed directly over the electron gun source to prevent debris from falling into the vicinity of the gun.
In addition to the beam deflection means, a second magnetic means, such as one or more electro magnets can be used to sweep the beam and thus to vary the location of the impact area of the beam. Heating irregularities caused by non-uniform electron density can be minimized by repetitive sweeping of the beam over the evaporant material.
Vapor sources which use a rotating crucible or a series of crucibles are also known. Use of a series of crucibles on a rotatable turret allows the evaporation of a series of different materials in one coating cycle. Use of one large rotatable crucible allows continuous evaporation of a large amount of material. If even longer evaporation cycles are desired, automatic loading devices can be used to replenish the material in the rotating crucible.
One important use of a cooled rotating hearth, such as will be described below, is in coating a heat sensitive substrate, such as a plastic part. In order to avoid softening of the plastic it is necessary to minimize the heat transferred to the substrate by radiation. This can be accomplished by loading the evaporant material in an annular pattern onto the rotating crucible at a controlled rate, and then completely evaporating the mixture as it passes through the electron beam impact area. Radiant heating of the substrate is minimized because the entire surface of the hearth is at a low temperature except for the small portion of the evaporant material in the impact area, and this portion is vaporized completely. Deposition of coatings by such a method of evaporation to dryness is disclosed in U.S. application Ser. No. 480,909 filed June 19, 1974, now abandoned. When a coating is deposited by this method, the composition of the evaporant material is duplicated in the coating, even if the composition is a mixture of components which have widely different vapor pressures and melting points. Thus, it is relatively easy to deposit a coating of an alloy of tungsten and aluminum, for example, which is very difficult to form by other methods.
Vapor sources have been developed which utilize a rotating crucible and a fixed electron-beam source with magnetic deflection and sweeping means. In previous apparatus of this type, the electron beam source and the deflection magnets were located outside the perimeter of the rotating crucible. This is a disadvantage because large pole pieces are then required even for a relatively small diameter rotating crucible.